Elucidation of the Photorhabdus temperata Genome and Generation of a Transposon Mutant Library To Identify Motility Mutants Altered in Pathogenesis

Genome Sequence Analysis of Native Xenorhabdus Strains Isolated from Entomopathogenic Nematodes in Argentina

Entomopathogenic nematodes from the genus Steinernema (Nematoda: Steinernematidae) are capable of causing the rapid killing of insect hosts, facilitated by their association with symbiotic Gram-negative bacteria in the genus Xenorhabdus (Enterobacterales: Morganellaceae), positioning them as interesting candidate tools for the control of insect pests. In spite of this, only a limited number of species from this bacterial genus have been identified from their nematode hosts and their insecticidal properties documented. This study aimed to perform the genome sequence analysis of fourteen Xenorhabdus strains that were isolated from Steinernema nematodes in Argentina. All of the strains were found to be able of killing 7th instar larvae of Galleria mellonella (L.) (Lepidoptera: Pyralidae). Their sequenced genomes harbour 110 putative insecticidal proteins including Tc, Txp, Mcf, Pra/Prb and App homologs, plus other virulence factors such as putative nematocidal proteins, chitinases and secondary metabolite gene clusters for the synthesis of different bioactive compounds. Maximum-likelihood phylogenetic analysis plus average nucleotide identity calculations strongly suggested that three strains should be considered novel species. The species name for strains PSL and Reich (same species according to % ANI) is proposed as Xenorhabdus littoralis sp. nov., whereas strain 12 is proposed as Xenorhabdus santafensis sp. nov. In this work, we present a dual insight into the biocidal potential and diversity of the Xenorhabdus genus, demonstrated by different numbers of putative insecticidal genes and biosynthetic gene clusters, along with a fresh exploration of the species within this genus.

Evaluation of the Antifungal Activities of Photorhabdus akhurstii and Its Secondary Metabolites against Phytopathogenic Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a phytopathogenic fungus that causes devastating losses in strawberries without effective countermeasures. Members of the genus Photorhabdus exhibit antimicrobial capability and have been found to have the potential for use as biocontrol agents against C. gloeosporioides. Photorhabdus species exhibit two phase variations with a differentiated composition of secondary metabolites designated to each phase. In this study, Photorhabdus akhurstii sp. nov. 0813-124 exhibited phase I (PL1) and phase II (PL2); however, only PL1 displayed distinct inhibition of C. gloeosporioides in the confrontation assay. We identified the bioactive ingredients of P. akhurstii sp. nov. 0813-124 to be glidobactin A and cepafungin I, with MIC values lower than 1.5 and 2.0 µg/mL, respectively. Furthermore, we revealed the biosynthetic gene cluster (BGC) of corresponding bioactive molecules through genomics analysis and determined its expression level in PL1 and PL2. The expression of glidobactin BGC in PL1 increased rapidly within 24 h, while PL2 was eventually stimulated after 60 h. In summary, we demonstrated that P. akhurstii sp. nov. 0813-124 could potentially be used as a biocontrol agent or part of a natural product repertoire for combating C. gloeosporioides.

Bacterial and Protozoan Lipoxygenases Could be Involved in Cell-to-Cell Signaling and Immune Response Suppression

Lipoxygenases are found in animals, plants, and fungi, where they are involved in a wide range of cell-to-cell signaling processes. The presence of lipoxygenases in a number of bacteria and protozoa has been also established, but their biological significance remains poorly understood. Several hypothetical functions of lipoxygenases in bacteria and protozoa have been suggested without experimental validation. The objective of our study was evaluating the functions of bacterial and protozoan lipoxygenases by evolutionary and taxonomic analysis using bioinformatics tools. Lipoxygenase sequences were identified and examined using BLAST, followed by analysis of constructed phylogenetic trees and networks. Our results support the theory on the involvement of lipoxygenases in the formation of multicellular structures by microorganisms and their possible evolutionary significance in the emergence of multicellularity. Furthermore, we observed association of lipoxygenases with the suppression of host immune response by parasitic and symbiotic bacteria including dangerous opportunistic pathogens.

Mortality Effects of Three Bacterial Pathogens and Beauveria bassiana When Topically Applied or Injected Into House Flies (Diptera: Muscidae)

The house fly, Musca domestica L., is a global pest of public health and agricultural importance. The efficacy of conventional management has been waning due to increasing insecticide resistance. A potential management tool is the entomopathogenic fungus, Beauveria bassiana Vuillemin (Hypocreales: Cordycipitaceae) (strain L90), although time-to-death is slower than desired by potential users. This research investigated the effectiveness of three gram-negative bacteria (Pseudomonas protegens Ramette (Psuedomonadales: Pseudomonadaceae) pf-5, Photorhabdus temperata Fischer-Le Saux (Enterobacteriales: Enterobacteriaceae) NC19, and Serratia marcescens Bizio (Enterobacteriales: Enterobacteriaceae) DB11) on house fly mortality when topically applied, compared to B. bassiana. Each pathogen's virulence was measured by injection into adult female house flies or by topical applications to their thorax. All bacterial strains were highly virulent after injection with 1 × 104 colony forming units (cfu), causing fly mortality within 24 h. Beauveria bassiana resulted in high mortality, 3 d postinjection at the high dose of 1 × 104 conidia/µl. Mortality due to topical treatments of P. temperata and S. marcescens was low even at the highest dose of 1 × 106 cfu/µl. Mortality after topical treatments with P. protegens was evident 4 d after application of 1 × 106 cfu/µl. Mortality from B. bassiana was low at 4 d but increased at 5 d. These results imply that P. protegens holds great potential as a biological control agent for incorporation into an integrated pest management program against adult house flies.

Permanent Draft Genome Sequence of Photorhabdus temperata Strain Hm, an Entomopathogenic Bacterium Isolated from Nematodes

Photorhabdus temperata strain Hm is an entomopathogenic bacterium that forms a symbiotic association with Heterorhabditis nematodes. Here, we report a 5.0-Mbp draft genome sequence for P. temperata strain Hm with a G+C content of 44.1% and containing 4,226 candidate protein-encoding genes.

Host Matters: Medicinal Leech Digestive-Tract Symbionts and Their Pathogenic Potential

Digestive-tract microbiota exert tremendous influence over host health. Host-symbiont model systems are studied to investigate how symbioses are initiated and maintained, as well as to identify host processes affected by resident microbiota. The medicinal leech, Hirudo verbana, is an excellent model to address such questions owing to a microbiome that is consistently dominated by two species, Aeromonas veronii and Mucinivorans hirudinis, both of which are cultivable and have sequenced genomes. This review outlines current knowledge about the dynamics of the H. verbana microbiome. We discuss in depth the factors required for A. veronii colonization and proliferation in the leech crop and summarize the current understanding of interactions between A. veronii and its annelid host. Lastly, we discuss leech usage in modern medicine and highlight how leech-therapy associated infections, often attributable to Aeromonas spp., are of growing clinical concern due in part to an increased prevalence of fluoroquinolone resistant strains.

Genome comparisons provide insights into the role of secondary metabolites in the pathogenic phase of the Photorhabdus life cycle

Background

Bacteria within the genus Photorhabdus maintain mutualistic symbioses with nematodes in complicated lifecycles that also involves insect pathogenic phases. Intriguingly, these bacteria are rich in biosynthetic gene clusters that produce compounds with diverse biological activities. As a basis to better understand the life cycles of Photorhabdus we sequenced the genomes of two recently discovered representative species and performed detailed genomic comparisons with five publically available genomes.

Results

Here we report the genomic details of two new reference Photorhabdus species. By then conducting genomic comparisons across the genus, we show that there are several highly conserved biosynthetic gene clusters. These clusters produce a range of bioactive small molecules that support the pathogenic phase of the integral relationship that Photorhabdus maintain with nematodes.

Conclusions

Photorhabdus contain several genetic loci that allow them to become specialist insect pathogens by efficiently evading insect immune responses and killing the insect host.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2862-4) contains supplementary material, which is available to authorized users.

Identification and characterization of Photorhabdus temperata mutants altered in hemolysis and virulence

Photorhabdus temperata is a symbiont of the entomopathogenic nematode Heterorhabditis bacteriophora and an insect pathogen. This bacterium produces a wide variety of virulence factors and hemolytic activity. The goal of this study was to identify hemolysin-defective mutants and test their virulence. A genetic approach was used to identify mutants with altered hemolytic activity by screening a library of 10 000 P. temperata transposon mutants. Three classes of mutants were identified: (i) defective (no hemolytic activity), (ii) delayed (delayed initiation of hemolytic activity), and (iii) early (early initiation of hemolytic activity). The transposon insertion sites for these mutants were identified and used to investigate other physiological properties, including insect pathogenesis and motility. The hemolysin-defective mutants, P10A-C11, P10A-H12, and P79-B5, had inserts in genes involved in RNA turnover (RNase II and 5'-pentaphospho-5'-adenosine pyrophosphohydrolase) and showed reduced virulence and production of extracellular factors. These data support the role of RNA turnover in insect pathogenesis and other physiological functions.

Draft Genome Sequence of Photorhabdus luminescens subsp. laumondii HP88, an Entomopathogenic Bacterium Isolated from Nematodes

Photorhabdus luminescens subsp. laumondii HP88 is an entomopathogenic bacterium that forms a symbiotic association with Heterorhabditis nematodes. We report here a 5.27-Mbp draft genome sequence for P. luminescens subsp. laumondii HP88, with a G+C content of 42.4% and containing 4,243 candidate protein-coding genes.

Flagellar Regulation and Virulence in the Entomopathogenic Bacteria-Xenorhabdus nematophila and Photorhabdus luminescens

There is a complex interplay between the regulation of flagellar motility and the expression of virulence factors in many bacterial pathogens. Here, we review the literature on the direct and indirect roles of flagellar motility in mediating the tripartite interaction between entomopathogenic bacteria (Photorhabdus and Xenorhabdus), their nematode hosts, and their insect targets. First, we describe the swimming and swarming motility of insect pathogenic bacteria and its impact on insect colonization. Then, we describe the coupling between the expression of flagellar and virulence genes and the dynamic of expression of the flagellar regulon during invertebrate infection. We show that the flagellar type 3 secretion system (T3SS) is also an export apparatus for virulence proteins in X. nematophila. Finally, we demonstrate that phenotypic variation, a common property of the bacterial symbionts of nematodes, also alters flagellar motility in Photorhabdus and Xenorhabdus. Finally, the so-called phenotypic heterogeneity phenomenon in the flagellar gene expression network will be also discussed. As the main molecular studies were performed in X. nematophila, future perspectives for the study of the interplay between flagellum and invertebrate interactions in Photorhabdus will be discussed.